1. Technical Field of the Invention
The present invention relates to voltage references and relates, more particularly, to a circuit for generating such a reference voltage.
2. DESCRIPTION OF RELATED ART
Accurate and stable reference voltages, in other words mostly independent of temperature, but also power supply voltages, are essential circuits for analog integrated circuits. In fact, the operational characteristics of active analog circuits are determined by the values of the reference voltages that they use. This is particularly the case for analog-digital or digital-analog conversion circuits whose resolution is directly linked to the stability of the reference voltage used in performing the conversion.
Today, various techniques exist for generating a stable reference voltage. As an example, reference could be made to the reference voltage generation circuits based on the principle of the bandgap energy, commonly denoted by those skilled in the art as bandgap reference circuits.
As can be seen in FIG. 1, which shows the general principle of such a circuit, the independence of the reference voltage with respect to temperature variations is based on the use of two diodes D1 and D2 which have different active areas and through which the same current flows. The cathode C of each diode is connected to the substrate Sub. The two diodes D1 and D2 are installed in two respective branches B1 and B2 of the circuit. These two branches are supplied by a voltage Vdd via an MOS transistor M1. A first branch, namely the branch denoted by the reference B1, has two resistors R1 and R3 placed in series between the source of the MOS transistor M1 and the anode of the diode D1. The second branch B2 has a resistor R2 identical to the resistor R1. An operational amplifier A imposes an identical voltage level between a first node N1 situated between the resistors R1 and R3 of the first branch B1 and a node N2 of the second branch B2 situated between the resistor R2 and the anode of the diode D2.
The resistors R1 and R2 are identical. As previously indicated, and by reason of the presence of the operational amplifier A, the currents flowing in the branches B1 and B2 are identical. The voltage equality between the nodes N1 and N2 imposes that:
                                                        I              ⨯              R                        ⁢                                                  ⁢            3                    +                                    KT              q                        ⁢            ln            ⁢                                                  ⁢                          I                              IS                ⁢                                                                  ⁢                1                                                    =                              KT            q                    ⁢          ln          ⁢                                          ⁢                      I                          IS              ⁢                                                          ⁢              2                                                          (        1        )            in which:                K denotes Boltzmann's constant        q is the charge on an electron        T is the operating temperature of the circuit in degrees K, and        IS1 and IS2 respectively denote the saturation currents of the diodes D1 and D2.        
This equation implies that:
                    I        =                                            1                              R                ⁢                                                                  ⁢                3                                      ⨯                          KT              q                                ⁢          ln          ⁢                                          ⁢          α                                    (        2        )            in which α is the ratio of the areas of the diodes D1 and D2.
The reference voltage delivered by this circuit is therefore given by the equation:
                    Vref        =                                                                              R                  ⁢                                                                          ⁢                  2                                                  R                  ⁢                                                                          ⁢                  3                                            ⨯                              KT                q                                      ⁢            ln            ⁢                                                  ⁢            α                    +                      Vd            ⁢                                                  ⁢            2                                              (        3        )            where Vd2 denotes the voltage across the terminals of the diode D2.
This equation (3) shows that the reference voltage may be considered as made up of the sum of two terms. One is proportional to temperature, whereas the diode voltage Vd2 is inversely proportional to it. By judiciously choosing the ratio R2/R3, a reference voltage that is virtually independent of temperature can be obtained.
However, as is known, in CMOS technology, diodes are fabricated using a base-emitter junction of a bipolar transistor. Such a transistor has a low gain. In addition, the collector is referenced to the circuit substrate. Thus, it will be understood that this type of circuit exhibits a certain number of major drawbacks, notably owing to the fact that, in the case where the substrate is affected by low- or high-frequency stray currents, these stray currents can propagate as far as the output of the circuit and affect the reference voltage level. In any case, the reference voltage is defined with respect to the substrate voltage, such that any voltage variation within the substrate results in a corresponding variation in the reference voltage.
This drawback can be redhibitory when it is desired to control a display screen of the LCD (Liquid Crystal Display) type in which the analog signals of the red, green and blue inputs are transformed into digital signals which are subsequently processed by algorithms designed for addressing the LCD matrices. The synchronization is effected using line synchronization pulses, either on a rising edge or on a falling edge. This pulse is relatively distorted, but the synchronization triggering must occur on very reproducible levels, whatever the temperature or the variations in power supply voltage. Moreover, the digital CMOS circuits used to control the LCD display screen tend to generate significant switching noise on the substrate. The synchronization is therefore effected by using hysteresis comparators with thresholds that are as independent as possible both of temperature and of the supply voltages.
In view of the above, there is a need to provide a reference voltage that is independent of temperature, but also of the voltage of the substrate on which the reference voltage generation circuit is built.